U.S. Pat. No. 5,267,905 (Wenzel et al from 1993) is an example of a common style of drive line, which has been used successfully for the past decade. This style of drive line will hereinafter be referred to as a circumferential ball drive line. In the circumferential ball drive line, balls are used to transfer torque between a central shaft and end housings. At each end, there are six or eight pockets in the exterior surface disposed around a circumference of the central shaft. Six or eight corresponding pockets are disposed within an interior surface of each of the end housings. By positioning balls in the respective pockets of the central shaft and the end housings, torque can be transferred, while accommodating omni-directional movement of the central shaft within the end housings.
In the past few years there has been a movement to longer power sections. As the rotors and stators in the power sections are lengthened, the power sections have produced greater torque. The torque output of the power sections has increased to the point that drive line failures are being experienced in circumferential ball drive lines.